Preparation of alkali metal borohydrides



United States PatentO PREPARATION OF ALKALI METAL BOROHYDRIDES NoDrawing. Application January 31, 1956 Serial No. 562,587

7 Claims. (Cl. 23 -14) This invention relates to a novel method for thepreparation of metal borohydrides. More particularly, it relates to theformation of metal borohydrides by the reaction of the correspondingmetal hydrides with the adduct of a lower tertiary aliphatic amine witha boron trihalide.

-T he alkali metal borohydrides are well known and very usefulcompounds. They are active reducing agents, advantageous sources ofdiborane and other boron hydrides and having many other uses. They havebeen produced in a variety of ways but all of these are subject to oneor more defects. Thus, the production of lithium borohydride fromlithium hydride and boron trifluoride etherate appears to be describedby the following equation:

The operation must be executed very cautiously to avoid formation ofdiborane. Another disadvantage is that theoretically one-fourth of thelithium hydride charged is converted to lithium borohydride andthree-fourths is converted to lithium fluoride. Both the lithium and thefluorine in the lithium fluoride are too expensive to be discarded. in alarge scale operation the necessary recycle operations are tremendous.

The prior art process requires the presence of ethyl ether which makesthe operation hazardous and unsuitable for large scale operations. Thisprocess is also not adaptable for the production of sodium borohydridein place of lithium borohydride. Simple substitution of sodium hydridefor lithium hydride and boron trichloride for boron trifluoride or itsetherate result in substantially no reaction to form borohydrides,diborane :or other boron hydrides. Thus the reaction represented by thefollowing equation:

4NaH+BCl NaBI-i +3NaCl does not appear to proceed.

The most successful industrial method for the preparation of sodiumborohydride is based on the reaction of sodium hydride withtrimethylborate. This reaction must be carried -out at elevatedtemperatures, necessitating the use of considerable pressures tomaintain the trimethylborate in the liquid state. At lower temperaturesand pressures, yields are poor. In addition, various by-produts areformed including sodium tetramethoxyborohydride and sodiumtrimethoxyborohydride.

The process of the present invention comprises the reaction of areactive metal hydride, for example, sodium, potassium or lithiumhydride with the adduct of a boron halide and a lower tertiary aliphaticamine. An example of the process of the present invention is shown inthe following equation:

solid reactants for about one to five hours in the presence of ahigh-boiling ether which will permit the use of reac- 2,880,058 PatentedMar. 31, 195 9- tion temperatures of about -200" C. without the ofpressure. Liberated trimethylamine is returned with out furthertreatment to reaction with additional quantities of boron trichloride toform the amine adduct starting material. Suitable amounts of the ethersolvent are used to dissolve all of the borohydride product. Filtrationof the reaction mixture serves to separate the by-product salt from thedissolved borohydride which is recovered by distillation of the etherfrom the residual borohydride. The by-product salt may be extracted torecover additional quantities of the borohydride using additionalquantities of the same ether or other solvent for the borohydride, forexample, isopropylamine. When such an additional extraction step ispracticed, the borohydride is recovered as a residual product bydistillation of the extracting agent which is recycled for further use.

Where the alkali metal halide is sodium chloride it may: be discarded.Using more expensive alkali metal hydrides' or more expensive boronhalides, it is advantageousto Work up the by-product salt for recoveryand recycle of the metal or halogen, or both.

Suitable ethers for use in the process of this invention be used whenthe solubility of the borohydride product is sutficient to permit itsseparation from the by-produot salt. It is a further particularadvantage of the dialkyl ethers'of polyalkylene glycols that theborohydride prod not frequently need not be isolated therefrom as thesolution is suitable for use in further reactions of the dissolvedborohydride as described in Walter J. Sakowski application Serial No.499,755, filed April 6, 1955.

Tertiary aliphatic amines suitable in combination with the boron halidefor formation of the adduct used as a starting material according to thepresent invention include trirnethylamine, triethylamine,tri-n-propylamine and mixed compounds, for example, rnethyldiethylamine.

Adducts of these amines with boron trifluoride, boron trichloride andboron tribromide, are useful starting materials for this process.

Suitable reactive metal hydrides for use according to the presentinvention include the alkali metal and alkaline potassium, calcium ormagnesium hydrides. A molar ratio of alkali metal hydride to amine-boronhalide adduct of 4:1 is required and the use of larger proportions ofthese hydrides is advantageous. The excess can be from about 10 to 50percent over the theoretical requirement.

Much of the excess hydride is recoverableand re-usable. When hydrides ofdivalent metals are used the minimum molar proportion is 2:1 and anexcess is similarly pre-' ferred.

A particularly advantageous combination of reactants is sodium hydrideand the adduct of trimethylamine with boron trichloride. In this casethe tertiary amine is the simplest and cheapest of the tertiaryaliphatic amines, boron trichloride is the cheapest boron halide andsodium hydride is the cheapest metal hydride. In addition, theby-product sodium chloride is of such low economic value that it can bediscarded without the necessity of working up the by-product alkalimetal halide for recovery of metal or halogen.

A further advantage of the process of the preferred example of thepresent invention is that the trimethylamine is readily separated as agas from the reaction mixture and recycled without the necessity forintermediate treatment to the formation of further quantities of thetrimethylamine-boron trichloride adduct. This adduct is a solid which issufiiciently stable in air to offer the advantage of ease of handling.

The preferred solvents of the present invention dissolve the resultingborohydride to a sufficient extent that the desired product can bereadily separated in solution from the insoluble alkali metal halide.Thus, sodium borohydride. is soluble in diethylene glycol dimethyl etherto theextent of about 9 percent by weight. The borohydride is stable.and has a very low pressure so that separation of the ether solvent bydistillation is very simple. It can be removed by distillation atatmospheric or reduced pressure. Substantially pure alkali metalborohydride is obtained as a residual product. Any residual high-boilingether in the solid borohydride can be readily removed by slurrying thesolid with an inert solvent for the highboiling ether, for example,ethyl ether, which is readily removed from the purified borohydride. Thewash solvent-can be removed, for example, by centrifuging and distillingfor reuse.

The reaction can be carried out at substantially atmospheric pressure orunder reduced or super-atmospheric pressure. Pressures not substantiallydifferent from atmospheric pressure are especially advantageous.

In the following examples the term moles signifies gram moles.

Example 1 A stainless steel reaction vessel having an internal volume of48 ml. was charged with 0.172 mole of finely powdered sodium hydride,0.0368 mole of trimethyl amine-boron trichloride adduct and about ml. ofdiethylene glycol dimethyl ether. The reaction vessel was closed,evacuated and shaken during a period of 2 hours whilethe contents weremaintained at 151-157 C. At the'end of the 2-hour heating period, thevessel and contents' werc allowed to cool to about 25 C. and opened,venting the trimethylamine. The residue was filtered at a temperature ofabout 25 C. to separate the insoluble sodium chloride and the ethersolution of sodium borohydride. Analyses showed that some borohydridewas present in both phases. The solution was evaporated to dryness andthe residue washed with diethyl ether. It was identified as sodiumborohydride by hydrogen and boron analyses which showed a ratio of3.88:1. The total yield of sodium borohydride was 24.5 percent.

Example II A stirred reaction vessel is charged with sodium hydride andtrimethylamine-boron trichloride adduct using a 20 percent excess ofsodium hydride over the theoretical molar ratio of 4:1 and sufiicientdiethylene glycol dimethyl ether to make a stirrable slurry of thereactants. The mixture is stirred and heated at a temperature of about150160 C. for 2 hours allowing trimethylamine to escape from thereaction vessel through a reflux condenser to a cold trap in which itcondenses. At this temperature the diethylene glycol dimethyl etherrefluxes slowly. After two hours the reaction mixture is cooled andfiltered, washing the by-product sodium chloride with additionaldiethylene glycol dimethyl ether to remove sodium borohydridesubstantially therefrom. The combined filtrates are evaporated,recovering the diethylene glycol dimethyl ether overhead and reducingthe solution to a slurry. The residue is filtered and washed withdiethyl ether to remove residual diethylene glycol dimethyl ether anddried to recover the pure sodium borohydride.

Example III A slurry of 10.9 grams (0.455 mole) of sodium hydride in 168grams of diethylene glycol dimethyl ether was stirred at C. while 14.2grams (0.0805 mole) of trimethylamine-boron trichloride adduct was addedover a period of 15 minutes. The reaction was continued for 30 minutesduring the last ten minutes of which a slow stream of nitrogen waspassed through the system. 'Trimethylamine amounting to 0.052 mole wasrecovered.- t

2. The method of claim 1 wherein said hydride is sodium hydride.

3. The method of claim 1 wherein said amine is trimethylamine.

4. The method of claim 1 wherein said boron trihalide is borontrichloride.

5. The method of claim 1 wherein said hydride is sodium hydride, saidamine is trimethylamine and said trihalide is borontrichloride.

6. The method of claim 1 wherein the reaction is conducted in a reactionmedium composed of at least one ether of the formula RO(C,,H ,,O),,,Rwherein n is an integer from 2 to 3, m is an interger from 2 to 4 andwherein R is an alkyl radical having not more than 4 carbon atoms.

7. The method of claim 6 wherein said ether is diethylene glycoldimethyl ether.

References Cited in the file of this patent UNITED STATES PATENTS2,741,540 Bragdon et al. Apr. 10, 1956 OTHER REFERENCES Burg et al.:J.A.C.S., vol. 62, pp. 3425-3429 (December 1940).

Sidgwick: Chemical Elements and Their Compounds, vol. 1, page 402(1950), University Press, Oxford.

Pryde July 27, 1954

1. A METHOD FOR THE PREPARATION OF A BOROHYDRIDE WHICH COMPRISES REACTING AN ALKALI METAL HYDRIDE WITH AN ADDUCT OF A LOWER TERTIARY ALIPHATIC AMINE AND A BORON TRIHALIDE WITHIN A TEMPERATURE RANGE FROM ABOUT 125* TO ABOUT 200*C. 